1. Field of the Invention
This invention relates to heart valves, and more particularly relates to replacement of diseased or injured heart valves.
2. Description of the Related Art
There are four valves in the heart that serve to direct blood flow through the two sides of the heart. On the left (systemic) side of the heart are: (1) the mitral valve, located between the left atrium and the left ventricle, and (2) the aortic valve, located between the left ventricle and the aorta. These two valves direct oxygenated blood from the lungs through the left side of the heart and into the aorta for distribution to the body. On the right (pulmonary) side of the heart are: (1) the tricuspid valve, located between the right atrium and the right ventricle, and (2) the pulmonary valve, located between the night ventricle and the pulmonary artery. These two valves direct de-oxygenated blood from the body through the right side of the heart and into the pulmonary artery for distribution to the lungs, where the blood becomes re-oxygenated in order to begin the circuit anew.
All four of these heart valves are passive structures in that they do not themselves expend any energy and do not perform any active contractile function. They consist of moveable “leaflets” that open and close in response to differential pressures on either side of the valve. The mitral and tricuspid valves are referred to as “atrioventricular valves” because they are situated between an atrium and ventricle on each side of the heart. The mitral valve has two leaflets and the tricuspid valve has three. The aortic and pulmonary valves are referred to as “semilunar valves” because of the unique appearance of their leaflets, which are shaped somewhat like a half-moon and are more aptly termed “cusps”. The aortic and pulmonary valves each have three cusps.
Heart valves may exhibit abnormal anatomy and function as a result of congenital or acquired valve disease. Congenital valve abnormalities may be well-tolerated for many years only to develop a life-threatening problem in an elderly patient, or may be so severe that emergency surgery is required within the first few hours of life. Acquired valve disease may result from causes such as rheumatic fever, degenerative disorders of the valve tissue, bacterial or fungal infections, and trauma.
Since heart valves are passive structures that simply open and close in response to differential pressures on either side of the particular valve, the problems that can develop with valves can be classified into two categories: (1) stenosis, in which a valve does not open properly, and (2) insufficiency (also called regurgitation), in which a valve does not close properly. Stenosis and insufficiency may occur concomitantly in the same valve or in different valves. Both of these abnormalities increase the workload placed on the heart. The severity of this increased stress on the heart and the patient, and the heart's ability to adapt to it, determine whether the abnormal valve will have to be surgically replaced (or, in some cases, repaired).
Valve repair and valve replacement surgery is described and illustrated in numerous books and articles, and a number of options, including artificial mechanical valves and artificial tissue valves, are currently available. However, the currently available options cannot duplicate the advantages of native (natural) heart valves. Some of the available mechanical valves tend to be very durable, but are problematic in that they are thrombogenic and exhibit relatively poor hemodynamic properties. Some of the available artificial tissue valves may have relatively low thrombogenicity, but lack durability. Additionally, even these artificial tissue valves often do not exhibit hemodynamic properties that approach the advantageous hemodynamic performance of a native valve. Some artificial tissue valves attempt to copy the form of native heart valves; such valves still fall short in durability and in hemodynamic performance.
James L. Cox, M. D. observed that during the natural embryological development, the human heart begins as a simple tubular structure, and changes its form during development based on its physiological function. Dr. Cox developed a tubular artificial heart valve, basing his research and development on the principle that “form follows function.” This principle can be restated for heart valves as: “if an artificial valve can be created that truly functions like a native valve, its resultant form will be very similar to that of the native valve.” The prosthetic heart valve that Dr. Cox developed based on this principle is discussed and disclosed in U.S. Pat. Nos. 5,480,424, 5,713,950 and 6,092,529. Each of these patents is hereby incorporated by reference in its entirety.
Dr. Cox's work has resulted in promising heart valve technology that can lead to the development of a prosthetic heart valve that can approach the overall performance of a native heart valve. Such a valve would be durable, nonthrombogenic, and would exhibit advantageous hemodynamics performance.